This invention relates to apparatus and a method for protecting an electric power system, in which a digital processing device such as a microcomputer is utilized and in which various values required for protecting the electric power system can be easily set.
In an ordinary protective relaying apparatus for protecting an electric power system, various values required for protecting the system were set beforehand by means of devices having setting taps, rotary switches, variable resistors and the like. More specifically, a distance relaying apparatus for protecting a transmission line can be made of a combination of one or more of over-current relays, undervoltage relays, mho relays, reactance relays, overvoltage relays, directional relays and the like, each requiring various values for its operation. As a result, the total number of setting devices required for such a distance relaying apparatus becomes extremely numerous, and furthermore since the values required for the protection of electric power transmission lines differ in accordance with the type of transmission line, the values must be specifically calculated and set by an operator.
Recently, progress in digital processing techniques have enabled a digital processing device such as a microcomputer to be widely used for the protection of power transmission systems and the like.
One of the advantageous features of digital protective relaying apparatus is that the functions of the various component protective relays described above can be achieved by a single digital processing device operated on a number of programs, and therefore the size of the protective relaying apparatus can be substantially reduced.
However, if a large number of setting devices is used in the protective relaying apparatus, an advantageous feature of reducing the size of the apparatus will not be fully realized.
This difficulty can be obviated by memorizing the large number of setting values which are required for the relaying apparatus in a memory device by means of a single input device. One procedure for realizing this feature utilizes a RAM (Random Access Memory) operable in both writing and reading modes, and another procedure utilizes a ROM (Read Only Memory).
The use of a RAM, however, is not practicable because the values memorized in a RAM will be lost in the event of a power failure, and the use of a ROM is not advantageous because a ROM, in the form of an IC (integrated circuit), requires a special writing device termed a "ROM writer", the operation of which is extremely complicated and which must be used each time rewriting the ROM is required.